In accordance with increasing demand for communication caused by the spread of the Internet and the like, a wavelength-division multiplexing (WDM) system utilizing the wide band of an optical amplifier is being developed.
Furthermore, in these years, a reconfigurable optical add-drop multiplexer (ROADM) system to be used in combination with the WDM system and a wavelength routing technique is being introduced into a metro ring network.
In addition, because demand for communicating audio data and a larger amount of image data is also increasing in accordance with the spread of fiber-to-the-home (FTTH) and the like, the communication rate per wavelength in a WDM system is increasing from approximately 10 Gbps to 40 Gbps, and even to approximately 100 Gbps.
In such an optical transmission system that communicates a large amount of data, a redundant configuration may be adopted in which light signals are transmitted using a plurality of transmission paths such as a work line and a protection line and received while switching the line in accordance with occurrence of failures, in order to improve the reliability during the occurrence of failures.
In an optical transmission system having a single ring network, for example, an optical protection architecture such as an optical unidirectional path-switched ring (OUPSR) or an optical shared path protection ring (OSPPR) is adopted.
FIG. 1 illustrates an example of the optical transmission system. In an optical transmission system 100 illustrated in FIG. 1, an optical transmitter station 200 divides a light signal generated with an optical transmitter 201 using an optical coupler 202, and outputs the divided light signal to a work line 401 and a protection line 402.
On the other hand, an optical receiver station 300 switches the connection thereof between the work line 401 and the protection line 402 using an optical switch (optical SW) 301 in accordance with occurrence of failures in the lines and the quality of communication, and receives a light signal. After amplifying the received light signal with an optical amplifier 302, the optical receiver station 300 extracts data from the light signal with an optical receiver 303. For example, as illustrated in FIG. 1, if a failure such as a disconnection of an optical fiber occurs in the work line 401, the optical receiver station 300 switches the connection thereof to the protection line 402 with the optical SW 301, and receives a light signal through the protection line 402.
FIG. 2 illustrates another example of the optical transmission system. An optical transmission system 101 illustrated in FIG. 2 is different from the optical transmission system 100 illustrated in FIG. 1 in that optical relays 501 and 502 are included in the work line 401 and the protection line 402, respectively. The other components 200, 201, 202, 300, 301, 302, and 303 are the same as those of the optical transmission system 100 illustrated in FIG. 1.
In the optical transmission system 101, too, if a failure such as a disconnection of an optical fiber occurs in the work line 401, the optical receiver station 300 switches the connection thereof to the protection line 402 using the optical SW 301, and receives a light signal through the protection line 402.
FIG. 3 illustrates an example of the WDM system. A WDM system 102 illustrated in FIG. 3, an optical transmitter station 200′ divides light signals generated with optical transmitters 201′-1 to 201′-n (“n” is an integer equal to or larger than 2) using optical couplers 202′-1 to 202′-n. The optical transmitter station 200′ then inputs the divided light signals to WDM couplers 203-1 and 203-2 to generate a plurality of wavelength-multiplexed light signals, and outputs the light signals to the work line 401 and the protection line 402.
A wavelength-multiplexed light signal transmitted through the work line 401 is amplified and relayed to the optical receiver station 300′ by a WDM optical relay 501′. A wavelength-multiplexed light signal transmitted through the protection line 402 is amplified and relayed to the optical receiver station 300′ by a WDM optical relay 502′.
On the other hand, the optical receiver station 300′ divides light signals having a plurality of wavelengths included in the wavelength-multiplexed light signals in units of wavelengths using WDM couplers 304-1 and 304-2.
Next, the optical receiver station 300′ switches the connection thereof between the work line 401 and the protection line 402 using optical SWs 301′-1 to 301′-n in accordance with occurrence of failures in the lines, and receives the divided light signals. For example, as illustrated in FIG. 3, if a failure such as a disconnection of an optical fiber occurs in the work line 401, the optical receiver station 300′ switches the connection thereof to the protection line 402 using the optical SWs 301′-1 to 301′-n, and receives light signals through the protection line 402.
The optical receiver station 300′ then amplifies the received light signals with optical amplifiers 302′-1 to 302′-n and extracts data from the light signals with optical receivers 303′-1 to 303′-n. 
FIG. 4 illustrates an example of the OADM system.
In an OADM system 103 illustrated in FIG. 4, an OADM node 600-1 divides light signals generated with optical transmitters (Tx's) 601-1 to 601-n using splitters (BSs) 602-1 to 602-n. 
The OADM node 600-1 inputs the divided light signals to WDM couplers 603-1 and 603-2 to generate a plurality of wavelength-multiplexed light signals.
A wavelength selection switch (WSS) 611 selects a desired wavelength of the wavelength-multiplexed light signal generated by the WDM coupler 603-1. The wavelength-multiplexed light signal is then amplified by an optical amplifier 612 and added to a work line 401-1. On the other hand, a WSS 617 selects a desired wavelength of the wavelength-multiplexed light signal generated by the WDM coupler 603-2. The wavelength-multiplexed light signal is then amplified by an optical amplifier 618 and added to a protection line 402-1.
As with the OADM node 600-1, OADM nodes 600-2, 600-3, and 600-4 have the configurations and the functions for transmission. For example, the OADM nodes 600-2, 600-3, and 600-4 may add light signals having desired wavelengths to work lines 401-2, 401-3, and 401-4 and protection lines 402-4, 402-3, and 402-2, respectively. Tx's 619-1 to 619-n, BSs 620-1 to 620-n, WDM couplers 621-1 and 621-2, WSSs 629 and 635, and optical amplifiers 630 and 636 of the OADM node 600-3 have the same functions as the Tx's 601-1 to 601-n, the BSs 602-1 to 602-n, the WDM couplers 603-1 and 603-2, the WSSs 611 and 617, and the optical amplifiers 612 and 618, respectively, of the OADM node 600-1.
On the other hand, the OADM node 600-1 amplifies a light signal received through the work line 401-4 using optical amplifiers 607 and 609 and compensates or mitigates the amount of chromatic dispersion using a dispersion compensator 608. The OADM node 600-1 then divides the light signal with the BS 610 and outputs the divided light signal to the WSS 611 and a WDM 606-1. The OADM node 600-1 also amplifies a light signal received through the protection line 402-4 using optical amplifiers 613 and 615 and mitigates the amount of the chromatic dispersion using a dispersion compensator 614. The OADM node 600-1 then divides (drops) the light signal with a BS 616 and outputs the divided light signal to the WSS 617 and a WDM 606-2.
Next, the OADM node 600-1 divides light signals included in the wavelength-multiplexed light signals in units of wavelengths using the WDM couplers 606-1 and 606-2 and outputs the divided light signals to optical SWs 605-1 to 605-n. 
The OADM node 600-1 then switches the connection thereof between the work line 401-4 and the protection line 402-4 using the optical SWs 605-1 to 605-n in accordance with occurrence of failures in the lines, and receives the light signals. Optical receivers (Rx's) 604-1 to 604-n extract data from the light signals.
For example, as illustrated in FIG. 4, if a failure such as a disconnection of an optical fiber occurs in the work line 401-4, the OADM node 600-1 switches the connection thereof to the protection line 402-4 using the optical SWs 605-1 to 605-n, and receives light signals through the protection line 402-4.
As with the OADM node 600-1, the OADM nodes 600-2, 600-3, and 600-4 have the configurations and the functions for reception. For example, the OADM nodes 600-2, 600-3, and 600-4 may drop light signals received through the work lines 401-1, 401-2, and 401-3 and the protection lines 402-3, 402-2, and 402-1, respectively. Rx's 622-1 to 622-n, optical SWs 623-1 to 623-n, WDM couplers 624-1 and 624-2, BSs 628 and 634, optical amplifiers 625, 627, 631, and 633, and dispersion compensators 626 and 632 of the OADM node 600-3 have the same functions as the Rx's 604-1 to 604-n, the optical SWs 605-1 to 605-n, the WDM couplers 606-1 and 606-2, the BSs 610 and 616, the optical amplifiers 607, 609, 613, and 618, and the dispersion compensators 608 and 614, respectively, of the OADM node 600-1.
In an optical receiver station of an optical transmission system, optical components whose optical losses are relatively large, such as an optical coupler and a WDM coupler adopting an arrayed waveguide grating (AWG), are used on a reception port side. In addition, in order to accommodate the increasing transmission rate of optical transmission systems, optical receivers have higher optical input levels. Therefore, an optical receiver station may include an optical amplifier that amplifies a received light signal.
FIG. 5 illustrates an example of the configuration of an optical receiver station used in a WDM system.
As illustrated in FIG. 5, an optical receiver station 300″ divides light signals included in wavelength-multiplexed light signals transmitted through a work line 401 and a protection line 402 in units of wavelengths using WDM couplers 304-1 and 304-2. Each wavelength-multiplexed light signal includes light signals having a plurality of wavelengths, and data has been superimposed upon each light signal by an optical transmitter station. From among the light signals divided using the WDM couplers 304-1 and 304-2, for example, light signals having a wavelength λ1 are input to BSs 305 and 306. Components light signals having other wavelengths are not illustrated in order to simplify the description.
The BS 305 divides a light signal input from the WDM coupler 304-1 and outputs the divided light signal to a photodetector (PD) 307 and an optical SW 301″. On the other hand, the BS 306 divides a light signal input from the WDM coupler 304-2 and outputs the divided light signal to a PD 308 and the optical SW 301″.
The PDs 307 and 308 judge whether the levels of the input light signals are equal to or higher than a threshold value, and notify the optical SW 301″ of results of the judgments. The optical SW 301″ detects occurrence of failures in the work line 401 and the protection line 402 on the basis of the results of the judgments made by the PDs 307 and 308, and switches, on the basis of detected failures, a source from which the optical SW 301″ receives light signals to connect the source to an optical receiver 302″.
For example, as illustrated in FIG. 5, if a failure such as a disconnection of an optical fiber occurs in the work line 401-4, the PD 307 judges that the level of an input light signal is lower than the threshold value and the optical SW 301″ detects occurrence of a failure in the work line 401 on the basis of the result of the judgment. The optical SW 301″ switches a path connected to the optical amplifier 302″ from the work line 401 to the protection line 402, and receives light signals through the protection line 402. Time taken until the optical SW 301″ completes the switching after occurrence of a failure is about 50 ms in most cases.
The optical amplifier 302″ amplifies a light signal from the optical SW 301″ and inputs the light signal to an optical receiver 303″. The optical receiver 303″ extracts data from the light signal amplified by the optical amplifier 302″.
At this time, the optical amplifier 302″ might be subjected to automatic level control (ALC) in order to cause the output level of the light signal after the amplification to be uniform. The control period of the ALC is, for example, about 1 kHz (about 1 ms) in most cases.
In this case, if a disconnection occurs in the work line 401, the optical SW 301″ switches the connection to the protection line 402, but no light signal is input to the optical amplifier 302″ (no-input state) after the occurrence of the disconnection until completion of the switching of the path.
When the optical amplifier 302″ enters the no-input state under the ALC, amplification gain is controlled such that the input level, which is close to zero at this moment, is amplified to a given output level, and therefore the amplification gain rises sharply. As a result, when transmission of light signals through the protection line 402 recovers upon the completion of the switching of the path performed by the optical SW 301″, an optical surge occurs in the optical amplifier 302″.
The optical surge in the optical amplifier 302″ may damage the optical receiver 303″. Furthermore, because of the occurrence of the optical surge, the optical receiver station 300″ might not be able to reproduce light signals.